Anisotropy Nanoindentation Properties of Human Cortical Bone

INTRODUCTION Cortical bone is an anisotropic material, and its mechanical properties are determined by its composition as well as its microstructure. On the microstructure level (a few microns), cortical bone is composed primarily of single osteons and interstitial lamellae. Nanoindentation has been shown to be an effective technique to probe mechanical properties of microstructures at the micron scale. However, for anisotropic materials, the indentation modulus obtained by nanoindentation in a specific direction is a composite quantity that depends on all of the elastic constants [1,3]. The purpose of this study is to investigate the effects of elastic anisotropy to nanoindentation measurements in cortical bone. MATERIALS AND METHODS A specimen was obtained from one human tibia (52 years old). After being dehydrated in a series of alcohol baths, 12 specimens were cut based on various surface orientations in order to assess the anisotropy of cortical bone. An orthogonal coordinate system was chosen, where directions 1, 2, and 3 were the radial, circumferential and axial directions, respectively. Nanoindentation was conducted in twelve different directions in the principal planes. After cutting, the specimens were embedded in epoxy resin at room temperature and pressure. Silicon carbide abrasive papers with decreased grit size (600, 800, and 1200 grit) were used to grind surfaces of specimens, then the specimens were polished on microcloths with 0.05 μm grit alumina powder. All nanoindentation tests (Nano Indenter II) were performed in single osteons or interstitial lamellae for each specimen. The position of each indent was carefully selected based on microscopy observations. A total of 216 indentions were conducted in three tests to minimize experimental errors. For anisotropic materials, the indentation modulus (M) can be defined by Eq. 1 [2], where s is contact stiffness and A is the projected area of contact. Based on a recently developed analysis of indentation of anisotropic